1. Field of the Invention
The present invention relates to a nonaqueous electrolytic secondary battery and a method of making the same, and, more particularly, to the improvement in charge and discharge cycling characteristics.
2. Description of the Related Art
With the recent advancement of electronics technology, small portable electronic devices such as camcorders, portable telephones, and laptop computers have been developed. In response to this, the development of a small and light secondary battery having high energy density as a portable power supply for the electronic equipment has been strongly demanded.
A nonaqueous electrolytic secondary battery that uses a light metal such as lithium, sodium, or aluminum as an active material for the negative electrode is expected as a secondary battery to meet the demand described above.
In theory, a nonaqueous electrolytic secondary battery can generate a high voltage and can have high energy density. Research and development have been active, in particular, on a nonaqueous electrolytic secondary battery using lithium as an active material for the negative electrode because of its high output and high energy density.
However, when a light metal such as lithium is used for a negative electrode as it is, the light metal is easily precipitated dendritically from the negative electrode during charging. Since the tip of the dendrite crystal has significantly high electric current density, the nonaqueous electrolytic solution decomposes, resulting in a decrease in cycling life, or the dendrite crystal precipitated from the negative electrode reaches the positive electrode, resulting in an internal short circuit of the battery.
In order to prevent such dendritical metal precipitation, instead of using a light metal as it is for the negative electrode, a carbonaceous material capable of being doped and undoped with light metal ions is used as a material for the negative electrode after the carbonaceous material is doped with the light metal ions.
As the carbonaceous material, graphite, cokes (e.g., pitch coke, needle coke, and petroleum coke), organic polymeric compounds (e.g., phenolic resin or furan resin that is at an appropriate temperature for carbonization), or the like is mainly used.
With respect to the nonaqueous electrolytic secondary battery that uses the carbonaceous material for the negative electrode, various improvements have been made in order to improve the charge and discharge cycling life and safety.
For example, one of the known factors which shorten the charge and discharge cycling life is decomposition of an electrolytic solution caused by direct contact between the electrode and the electrolytic solution. In order to prevent such decomposition of the electrolytic solution, Japanese Patent Laid-Open Nos. 4-22072, 7-134989, and 6-22282 disclose batteries, in which negative electrodes are coated with polymeric materials so that the electrodes and the electrolytic solutions do not come into direct contact with each other.
Also, in accordance with Japanese Patent Laid-Open No. 7-192753, in a battery using a wound electrode member in which a strip negative electrode and a strip positive electrode laminated with a separator therebetween are wound up in a coil, by inserting a polymer core in the center of the wound electrode member, a rapid rise of the battery temperature during an external short circuit is prevented.
However, the above-mentioned methods will cause the problems described below.
First, in order to fabricate a negative electrode coated with a polymer, a conductive polymer is coated on the surface of the negative electrode, or carbonaceous particles as a material for a negative electrode are treated with a solution in which a polymer is dissolved to coat the surface of the particles and a negative electrode is fabricated using the carbonaceous particles.
However, when the surface of the negative electrode is coated with the polymer in such a manner, the fabrication process of the negative electrode becomes complex, resulting in low productivity of a battery. Also, in accordance with the methods described above, it is difficult to control the volume of the polymer to be coated, and if the volume of the polymer to be coated is too high, load characteristics and capacity characteristics of the battery are damaged.
With respect to a battery in which a polymer core is inserted in the center of the wound electrode member, the volume occupied by the core is not involved in the battery reaction, and thus the energy density per battery volume decreases.
Accordingly, the present invention has been made in view of the situation described above, and it is an object of this invention to provide a nonaqueous electrolytic secondary battery, in which charge and discharge cycling characteristics are improved without damaging load characteristics or capacity characteristics, and a temperature rise owing to an external short circuit can be suppressed, and to provide a method of making such a nonaqueous electrolytic secondary battery.
A nonaqueous electrolytic secondary battery in accordance with the present invention includes a negative electrode, a positive electrode, and a nonaqueous electrolytic solution in which an electrolytic salt is dissolved in a nonaqueous solvent, and a polymer is added to the nonaqueous electrolytic solution.
A method of making a nonaqueous electrolytic secondary battery in accordance with the present invention includes the steps of placing a negative electrode, a positive electrode, and a nonaqueous electrolytic solution in which an electrolytic salt is dissolved in a nonaqueous solvent, in a battery housing to assemble the battery, and charging and discharging the battery under overcharge conditions.
Alternatively, a method of making a nonaqueous electrolytic secondary battery in accordance with the present invention includes the steps of placing a negative electrode, a positive electrode, and a nonaqueous electrolytic solution in which an electrolytic salt is dissolved in a nonaqueous solvent, in a battery housing to assemble the battery, and applying a pulse voltage to the battery.
In the nonaqueous electrolytic secondary battery, by adding the polymer to the nonaqueous electrolytic solution, decomposition of the nonaqueous electrolytic solution during charging and discharging is suppressed, and thus charge and discharge cycling characteristics are improved. Also, when the battery temperature is rapidly raised by an external short circuit, the polymer in the nonaqueous electrolytic solution decomposes by absorbing heat, and thus the temperature rise of the battery is relieved.
Also, after assembling the nonaqueous electrolytic secondary battery, by charging and discharging under overcharge conditions or by applying a pulse voltage to the battery, a reaction occurs among a portion of the components in the nonaqueous electrolytic solution to generate a polymer. When the polymer is generated in the nonaqueous electrolytic solution in such a manner, decomposition of the nonaqueous electrolytic solution is suppressed during subsequent charging and discharging, and thus charge and discharge cycling characteristics are improved. Also, when the battery temperature is rapidly raised by an external short circuit, the polymer in the nonaqueous electrolytic solution decomposes by absorbing heat, and thus the temperature rise of the battery is relieved.